In photocomposing a graphic pattern is prepared on or composed on a photo-sensitive member such as a photographic film or the like. This record may be artistic, informational or may be utilized for a practical intended purpose. A common use and application is in phototypesetting where it is usual to prepare a photo record from a store of preformed alphanumeric characters on a rotatable film record and to rotate this record at high speed. The desired character is selected and its motion stopped with a timed flash of light and the image of the character is thus projected onto the photocomposition recording film. It was with systems such as this that phototypesetting reached widespread commercial use.
More recently other faster or more flexible means of character formation or pattern writing have been introduced. In one line of effort the laser became common as a light source, being a fast, high intensity light source. In another line of effort character formation and pattern writing replace the preformed alphanumeric type font. Tidd U.S. Pat. No. 4,270,172 is illustrative, showing a cathode ray tube as an image former to produce a light pattern representing the desired alphabet, numbers and graphic patterns to be set as a printing master.
The cathode ray tube system solves many problems in producing a printing master, but it, also, is in need of improvement. There are problems and limitations with such systems, particularly in making high quality, high resolution images, such as the difficulty of maintaining precise positioning of large images and large image segments on the face of the tube. Particularly where an image has continuing lines through a number of image segments through a number of scans, it is difficult to maintain the precise positioning throughout the length of a line or across the individual scan segments. In addition there is the well known tendency of cathode ray tube systems to drift, particularly with changes in temperature or upon aging of system components. The need, therefore is clear for a system to retain many of the advantages of the cathode ray tube system while avoiding its difficulties.
Some but not all of the problems are alleviated by a system such as that disclosed in Ebner U.S. Pat. No. 4,342,504 which uses a narrow printing head with several light emitting diodes (LEDs) whose output is carried to a photorecording element by a bundle of glass fibers. One trouble with systems using LEDs is that the LEDs are not uniform in light emitting quantity, with the result that the record also is not uniform in density. For example, at the present state of the art of manufacture of LEDs in large quantities it is common to have variations of at least as great as double or half the quantity of light emitted in response to a signal applied to one or another of such LEDs.
The present invention has solved these and other problems and employs light emitting devices such as light emitting diodes which may be of nonuniform quantity of emission, still achieving a uniform image on a photorecording element and avoiding many of the problems associated with prior art systems of photocomposing.